Commercially available synthetic bone grafts are usually made of calcium phosphate ceramics (the main inorganic materials of human bone) and have a porous structure similar to the human cancellous bone. Many of them are actually derived from animal (young bovine) or marine (sea coral) life. They are intended to offer an interconnected macroporous structure and provide intensive osteoconductivity to regenerate and heal the host bone tissue. However, none of them offer the biomechanical and osteointergrative properties equivalent to the gold standard of autograft.
These synthetic bone grafts usually come with an interconnected macroporous structure, typically of 100˜500 μm diameter, which provides a framework for the host bone to regenerate while reducing healing time. The pore size of the porous structure is crucial for the osteoconductivity. According to the in vitro and in vivo experiments, the proper pore size for bone tissue ingrowth is around 200˜300 μm. If the pore sizes are smaller than 100 μm the bone tissue may accumulate on the surface without osteoingrowth. After the implant, the bone graft should be slowly degraded and replaced by the growing bone. It should result in bone replacement at the site of defective bone by the recipient's own osteogenic activity. However, degradation requires the bone substitute materials to be microporous, with pore diameter from 1˜5 μm. The dissolution process of the “degradable” bone graft occurs in two steps: extracellular dissolution of the necks among sinterized particles, and intracellular phagocytosis of the particles isolated in this way. The first step becomes impossible in annealed bioceramics bulk and very difficult in those porous synthetic bone grafts with a thick connected wall because there are no small necks that the cells can attack.
Commercially available synthetic bone grafts usually have a random distribution of pore sizes and no observable preferred orientation of the inter-connected porous structure. The structure has the potential to prevent vascularisation after a period of time in vivo and the middle of the bone graft usually remains bone free. Although most of the commercial bone grafts have a similar chemical composition to the mineral phase of the living bone, the graft is not suitable for large scale application or as the permanent replacement since nutrients cannot flow through the synthetic porous bone graft after the surgery.